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One way to characterize the health of lakes is by using phosphorus, chlorophyll a, and secchi depth transparency data to calculate the Trophic State Index (TSI, Carlson 1977). This index provides a way to rate and compare lakes according to their level of biological activity on a scale from 0 to 100. As the TSI values increase by 10 (10, 20, 30, etc.) they represent a doubling of algal biovolume that can be related to easily measured parameters through linear regression and re-scaling. The TSI scale provides thresholds for three ranges of lake primary productivity (oligotrophic, mesotrophic, and eutrophic) as shown in Table 1.

Average Summer (June-September) Trophic State Index Values

TSI Value Classification Characterization
< 40 Oligotrophic High water clarity, low chlorophyll concentrations and phosphorus concentrations representing low biological activity.
40-50 Mesotrophic Moderate water clarity, chlorophyll, and phosphorus concentrations representing medium biological activity
> 50 Eutrophic Low water clarity, with high chlorophyll and phosphorus concentrations, representing high biological activity.

The indices are based on summer mean values (June through September) of three commonly measured lake parameters: water transparency measured by Secchi depth, and concentrations of total phosphorus and chlorophyll-a in the epilimnion (upper 10 meters). TSI values were calculated for Lake Union (A522), Lake Washington (Station 0852), and Lake Sammamish (0612) as shown Figures 1a - c.

union-tsi wash-tsi


The relationships between the three TSI calculations are not always straightforward. Carlson points out that highly colored lakes containing large amounts of dissolved organic matter may produce high TSI ratings for Secchi transparency that don’t fit the model because the depth at which the Secchi disk disappears is impacted by water color. The shape and size of dominant phytoplankton species can also influence the Secchi reading, as well as the chlorophyll values, since small, diffuse algae cloud the water more than large, dense algal colonies, and the species of algae can vary in the amount of chlorophyll they contain.

Carlson (1977) points out that if all the assumptions are correct, the TSI values produced for the three different parameters should be very close to each other. If lakes produce two close TSI values and one very different one, the outlying value might be excluded from consideration if a reasonable explanation is put forward for the differing value. For lakes Union, Washington, and Sammamish TSI values calculated for the three parameters tend to fluctuate in unison showing similar year-to-year patterns. TSI values for Secchi transparency were generally lower than either TSI-TP or TSI-chla indicating that water clarity in the open deep areas of these lakes is influenced less by phosphorus and subsequent algal growth than predicted by Carlson’s formula, particularly in Lake Union. This suggests that some conditions may exist in Lake Union that are not covered by Carlson’s assumptions when he set up the TSI calculations.

TSI Status

In general the TSI-TP values for Lake Sammamish and Lake Washington fluctuate below the low to moderate threshold indicating fairly consistent low phosphorus with low potential for nuisance algal blooms. Lake Union TSI-TP values were mostly within the moderate water quality range. NOTE: The King County Environmental Lab changed total phosphorus analysis method in July 1998 and again in Jan 2007. These changes resulted in a bias towards lower TP values reported. Therefore to make comparisons with recent data, values reported before July 1, 1998 were divided by a factor of 1.262. (R2 = 0.989) for TSI calculations.

Lake Sammamish is the only one of the three lakes with a management plan and designated water quality goals. The plan calls for an annual volume-weighted total phosphorus concentration (VWTP) of 22 µg/L or less. To see how the lake is meeting this goal, click on the Lake Sammamish link.

King County will continue to monitor these lakes as part of the ongoing Major Lakes Ambient Monitoring Program. This program is designed to track how lakes respond over time to various activities and inputs from the watersheds through influent streams, lake nutrient cycles, ecological interactions, and seasonal or year-to-year variability in weather.

Secchi Depth Transparency

Secchi depth transparency is a measurement of water clarity produced by lowering a Secchi disk --- an 8-inch disk with alternating black and white quadrants --- into the water until the observer can no longer see it. This depth of disappearance, called the Secchi depth, is a relative measure of the water’s transparency that can be used to look at events in a lake, trends over time, or make comparisons between lakes. Algae, soil particles, and other materials suspended in the water all affect transparency. The Secchi depth will decrease as these factors increase. In King County, clarity tends to be lower during periods of high algal growth (spring and summer) and during periods of high stormwater flows (winter).


Nutrients such as nitrogen, phosphorus, and silica are necessary for plant and animal growth. However, increasing nutrients availability can increase the growth of aquatic plants, which can cause nuisance blooms that subsequently decay. Decomposition can deplete oxygen to levels incapable of sustaining many aquatic organisms, thus leading to more problems. In the temperate latitudes, phosphorus is most often the primary nutrient of concern in freshwater systems because it is usually the nutrient that is in shortest supply, thus limiting algae growth. If excess phosphorus gets into lake water, it can cause nuisance algal blooms or even algal blooms that produce toxins. Additional phosphorus from human activities enters water bodies via pathways such as discharge of detergents, runoff containing fertilizers, pet waste, car washing and seepage from failing septic systems. Sediment can also be a source of phosphorus, as phosphorus readily binds to soil particles and is washed into the lakes. Through chemical reactions, phosphorus may be later released into the water column from the lake sediments when DO concentrations fall below 0.2 mg/L.


Chlorophyll is the green pigment in plants that allows them to create energy from light (photosynthesis). Measuring chlorophyll provides an indirect estimate of the amount of algae in the water column. Chlorophyll-a is a measure of the portion of the pigment that is still actively photosynthesizing at the time of sampling. There are several other forms of chlorophyll present in different groups of algae, as well as other assisting pigments and degradation byproducts that may be found in the water, but chlorophyll-a is the most common form present and can be used as an indicator of the volume of algae present

Factors Influencing TSI Values

The more phosphorus that can be prevented from entering lakes, the less chance that a potentially toxic cyanobacteria bloom will occur. The cyanobacteria generally responsible for making toxins are known to be poor competitors for phosphorus, so the available levels must be high before they will do well in a body of water. Phosphorus inputs can be minimized through well-designed storm water drainage systems, maintenance of sewer infrastructure, changing homeowner and business behaviors (such as using no phosphorus-rich fertilizers on lawns), education and incentives, and replacing watershed septic systems with sewers.


Carlson, Robert E. 1977. A trophic state index for lakes. Limnological Research Center, University of Minnesota, Minneapolis 55455